1. Field of the Invention
This invention relates to AM stereo receivers designed for reception of a carrier wave having stereo related intelligence appearing in the respective upper sideband and lower sideband of the transmitted signal such as disclosed in my prior U.S. Pat. No. 3,218,393, and in my aforesaid copending application Ser. No. 487,155 entitled Compatible AM Stereophonic Transmission System. An infrasonic frequency (e.g. 15 Hz) modulation of the carrier wave is preferably incorporated in the transmitted signal for indication in the receiver of the presence of a stereo modulated signal. Aspects of the invention also relate to specialized receiver circuitry with both stereophonic mode and monophonic mode capabilities and automatic switching therebetween.
2. Description of the Prior Art
Compatible stereophonic AM transmission and reception, involving stereo related upper and lower sidebands, with the difference stereo signal (channel A minus channel B) phase modulating the carrier wave and with the summation stereo signal (channel A plus channel B) envelope modulating the carrier wave, are disclosed in my U.S. Pat. No. 3,218,393, together with certain forms of receivers for stereophonic reception of a carrier wave so modulated. A further discussion of this compatible AM stereophonic modulation technique appears in my paper entitled "A Stereophonic System For Amplitude Modulated Broadcast Stations", which appears in IEEE Transactions on Broadcasting, Vol. BC-17, No. 2, June 1971, at pages 50-55. To the extent here relevant, the disclosures of this prior patent and this paper are incorporated herein by reference.
Also known are stereophonic transmission and reception systems as disclosed in Shoaf U.S. Pat. No. 3,009,151, involving a two-channel FM - AM stereo system wherein stereo related signals are respectively frequency modulated and amplitude modulated on FM band and AM band carrier waves; Colodny U.S. Pat. No. 3,031,529, disclosing a single channel AM stereo system employing synchronous detectors in the receiver portion of the system; Avins U.S. Pat. No. 3,068,475, disclosing a stereo transmission and reception system wherein one stereo related signal is amplitude modulated on a carrier wave and the other stereo related signal is frequency modulated on the same carrier wave; Barton U.S. Pat. No. 3,102,167, disclosing a two-channel, phaseshifted, double sideband stereo transmission; Fink U.S. Pat. No. 3,206,550, disclosing visual display of a stereo presence signal; Holt et al. U.S. Pat. No. 3,167,614, disclosing use of an infrasonic tone to indicate stereo signal presence in an AM/PM type transmission system; and Collins U.S. Pat. No. 3,231,672, disclosing an AM stereo system involving linearly added carrier waves at the same frequency but in different phase, with each of the carrier waves amplitude modulated with stereo related signals.
Also known is a system for transmission of stereophonic signals over telephone lines, as in Almering et al U.S. Pat. No. 3,803,490, granted June 3, 1974, wherein two different carrier frequencies are employed with a relatively wide bandwidth requirement (e.g. 65 kHz to 103 kHz with 8.06 kHz break), and with no attempt to make the system compatible from the point of view of detection of signals by envelope detection means.
Also notable as being of general interest, in the field of CSSB and stereophonic signal transmission are the following:
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